Control moment gyroscopes are well known means of providing directional control for a variety of vehicles, most notably spacecraft. Control moment gyroscopes normally comprise a rotor, a motor to spin the rotor about a rotor axis, a gimbal, a gimbal torque motor to rotate the gimbal about a gimbal axis, and a control system. The control moment gyroscope is mounted within the spacecraft along the axis in which it will in induce a torque. The rotor is mechanically supported in the gimbal and is rotated about the gimbal axis which is normal to the rotor axis.
During operation of the gyroscope, the rotor is spun by a motor about its rotor axis at a predetermined rate. In order to induce a torque on the spacecraft, the gimbal torque motor rotates the gimbal and spinning rotor about the gimbal axis. The rotor is of sufficient mass and is spinning at such a rate that any movement of the rotor out of its plane of rotation will induce a significant torque around an output axis which is both normal to both the rotor axis and the gimbal axis. Torque around the output axis is transferred directly to the spacecraft.
One requirement of a control moment gyroscope is that the torque to the spacecraft be delivered in as smooth a fashion as possible. In the prior art gyroscopes, anomalous torques can be caused by rate variations in the rotation of the gimbal. These anomalous torques, or ripple torques, can become significant in light of the fact that they are directly proportional to the angular momentum of the rotor. So the greater the mass and rate of rotation of the rotor, the greater the ripple torques can be.
The ripple torques are caused from a variety of sources. They may include gimbal rate sensor (tachometer) errors, motor ripple torque, motor cogging torque, motor gear train imperfections, electronic drive amplifier biases, electronic drive amplifier gain variations, motor commutation offsets, motor commutation gain variations, and gimbal bearing noise. The common thread running through all these potential problem sources is that they are all related to the operation of the gimbal torque motor.
Thus, there is need for a control moment gyroscope that delivers torque to the spacecraft in a smooth fashion and substantially eliminates ripple torques.